In .NET 2.0 there is a cool class called SmtpClient within System.Net.Mail.

With this class you can send mail (at least I thought).
The requirement was to send an email over a server of one of our agencies.
The mail server was secured by username/password authentication. That's why I used the following code:

SmtpClient _client = new SmtpClient();
_client = new SmtpClient("");

_client.UseDefaultCredentials = false;
_client.Credentials = new NetworkCredential("username", "password");

_client.Send("", "", "Hallo Welt", "Hallo max!!");
At execution of the last statement an exceptíon occurred:
The inner exception said "Invalid lenght for a Base-64 char array."
That didn't make any sense for me (nor helped me to fix the problem).
I had a look at the stack trace and found out, that the exception had occurred in a class called


So I used telnet to manually connect to the smtp server.

S: 220 ESMTP
C: ehlo
S: Hello []
S: 250-SIZE 20971520
C: AUTH NTLM <base64 encoded string>

S: 334 NTLM supported

Seemed like the server would support NTLM.. But.. when I looked up the NTLM-SMTP specification,I found out that the server should respond with

334 <NTLM supported as base64 encoded string>

So the problem obviously is, that "NTLM supported" was not a valid Base-64 encoded string (as the inner exception above also pointed out).

So how could this problem be solved...

I digged into the private members of the SmtpClient object and found a member called transport (of type SmtpTransport).

The SmtpTransport object had private members as well, and one of them was called authenticationModules - bingo!

This is an array of ISmtpAuthenticationModules like Negotiate, NTLM, Digest and Login.

Unfortunately SmtpClient always picks the most effective supported method (in this case NTLM). As NTLM was not working I needed a way to kick out NTLM of the list of supported auth methods.

So I used reflection to modify the array and "disable" (override) NTLM in the array. Here's what I did:

FieldInfo transport = _client.GetType().GetField("transport",
    BindingFlags.NonPublic | BindingFlags.Instance);

FieldInfo authModules = transport.GetValue(_client).GetType()
        BindingFlags.NonPublic | BindingFlags.Instance);

Array modulesArray = authModules.GetValue(transport.GetValue(_client)) as Array;
modulesArray.SetValue(modulesArray.GetValue(2), 0);
modulesArray.SetValue(modulesArray.GetValue(2), 1);
modulesArray.SetValue(modulesArray.GetValue(2), 3);


Guess which smtp authentication module will be used now :-)